This application is a National Stage application of International Application No. PCT/JP2012/073192, filed Sep. 11, 2012.
1. Technical Field Text
The present invention relates to an information recording medium glass substrate and an information recording medium.
2. Background Art
An information recording device such as an HDD (Hard Disk Drive) contains a disk-shaped information recording medium such as a magnetic disk or an opto-magnetic disk. A disk-shaped information recording medium glass substrate is used for the information recording medium. A magnetic thin film layer including a recording layer having magnetic, optical, or opto-magnetic properties is formed on a main surface of the information recording medium glass substrate. The magnetic thin film layer is magnetized by a magnetic head whereby prescribed information is recorded in the information recording medium.
The information recording medium rotates at high speed in the inside of the information recording device such as an HDD. To record and reproduce information into/from the information recording medium, the magnetic head flies over the magnetic thin film layer formed on the main surface of the information recording medium glass substrate. A DFH (Dynamic Flying Height) mechanism is employed in information recording devices in order to increase the recording density.
The distance (hereinafter referred to as “floating height”) between the magnetic head and the outermost surface (the surface of the magnetic thin film layer) of the information recording medium has been over 10 nm. Letting the radius length from the center of the information recording medium to the outer edge in the radial direction be 100%, even when the head travel region is extended to a region over 97% from the center, there has been no problem in write and read of information into/from the information recording medium by the magnetic head.
With the recent increasing recording density, there has been a tendency for the floating height to be smaller. Reducing the floating height can improve the S/N ratio of a received signal at the magnetic head and increase the recording density in the information recording medium.
With the floating height of 5 nm or less, demand for smoothness and flatness of the main surface of the information recording medium glass substrate is increasing in order to prevent the contact between the outermost surface of the information recording medium and the magnetic head (this phenomenon is called a head crash). When entering the recording surface of the information recording medium from the retracted position, the magnetic head is likely to be affected by air flow disturbance resulting from the shape of the outer peripheral end surface of the information recording medium, possibly leading to unstable floating of the magnetic head (glide avalanche).
The unstable floating of the magnetic head during transition from the retracted position to the travel position on the recording surface of the information recording medium has adverse effects on the floating of the magnetic head during travel on the recording surface. Consequently, the recording density on the information recording medium cannot be increased efficiently.
Japanese Laid-Open Patent Publication Nos. 2011-000704 (Patent Document 1) and 2007-257811 (Patent Document 2) describe that flatness in the neighborhood of the outer edge of an information recording medium is also required in the same manner as the flatness of the surface of the information recording medium in order to increase the amount of information that can be stored in a single information recording medium.